Abstract. Aerosol mass and the absorbing fraction are important variables, needed to
constrain the role of atmospheric particles in the Earth radiation budget,
both directly and indirectly through CCN activation. In particular, their
monitoring in remote areas and mountain sites is essential for determining
source regions, elucidating the mechanisms of long range transport of
anthropogenic pollutants, and validating regional and global models. Since
March 2006, aerosol mass and black carbon concentration have been monitored
at the Nepal Climate Observatory-Pyramid, a permanent high-altitude
research station located in the Khumbu valley at 5079 m a.s.l. below Mt.
Everest. The first two-year averages of PM1 and PM1−10 mass were
1.94 μg m−3 and 1.88 μg m−3, with
standard deviations of 3.90 μg m−3 and
4.45 μg m−3, respectively, while the black carbon
concentration average is 160.5 ng m−3, with a standard deviation of
296.1 ng m−3. Both aerosol mass and black carbon show well defined
annual cycles, with a maximum during the pre-monsoon season and a minimum
during the monsoon. They also display a typical diurnal cycle during all the
seasons, with the lowest particle concentration recorded during the night,
and a considerable increase during the afternoon, revealing the major role
played by thermal winds in influencing the behaviour of atmospheric
compounds over the high Himalayas. The aerosol concentration is subject to
high variability: in fact, as well as frequent "background conditions"
(55% of the time) when BC concentrations are mainly below 100 ng m−3,
concentrations up to 5 μg m−3 are reached during some
episodes (a few days every year) in the pre-monsoon seasons. The variability
of PM and BC is the result of both short-term changes due to thermal wind
development in the valley, and long-range transport/synoptic circulation. At
NCO-P, higher concentrations of PM1 and BC are mostly associated with
regional circulation and westerly air masses from the Middle East, while the
strongest contributions of mineral dust arrive from the Middle East and
regional circulation, with a special contribution from North Africa and
South-West Arabian Peninsula in post-monsoon and winter season.